The invention is directed to a process for the production of plate-shaped fuel elements for research and material testing reactors having loads of more than 26 volume % uranium compounds, by mixing the powdered uranium compounds with aluminum powder, molding to a plate-shaped molded body, enclosing the molded body in an aluminum jacket, and inserting it into an aluminum plate having a cutout corresponding to the molded body, covering the molded body and the aluminum plate on both sides with two aluminum sheets and rolling this packet.
It is known to produce plate-shaped fuel elements for research and material testing reactors according to the so-called picture frame technique. Thereby, first the actual fuel nucelus of this fuel element is produced first from a mixture of suitable fuel powder with aluminum powder in the form of a flat platelet, which customarily is called a picture. This picture is fitted into a frame of comparable thickness and covered above and below with two sheets. Frame and covering sheets are manufactured of aluminum or aluminum alloys. The sandwich-like "packet" formed is welded locally laterally and then rolled out in several rolling passes to a plate. Consequently, the finished fuel element consists of an aluminum or aluminum alloy plate which in the interior encloses a fuel containing zone made of a uranium compound in an aluminum matrix. Corresponding to the construction of this fuel zone, there is talk of dispersion nuclear fuels. As fuels, uranium compounds such as the oxide, silicide, aluminide, or ferride are employed.
The construction of such plate-shaped fuel elements is carried out in each case depending on the requirements of the research or material testing reactor to be loaded. Depending on the capacity of the reactor, the uranium content must be higher or lower in the fuel zone. With the customary standard fuel elements which are produced with insertion of highly enriched uranium, the portion of uranium compound in the fuel zone has a value between 7 and 26 volume percent.
However, the reactors and the corresponding fuel elements can also be changed over to receive lower enriched uranium. While highly enriched uranium contains 80 to 93 weight % of the fissionable isotope U-235, the U-235 portion of lower enriched uranium shrinks to a maximum of 20 wt.%.
To maintain the reactor capacity in the change over from highly enriched uranium to fuel elements with lower enriched uranium, the uranium density in the fuel must be raised correspondingly. With a reactor having a lower capacity, this leads to a raising of the uranium density in the fuel zone, e.g., from 0.5 to 2.4 grams U/cm.sup.3, with a reactor of higher capacity from, e.g., 1.3 to 7.0 grams U/cm.sup.3.
If the uranium compound portion of a dispersion fuel is limited to 26 vol.% because of manufacturing reasons, then even with insertion of high density uranium fuels, such as U.sub.3 Si (density 15.6 g/cm.sup.3) in the fuel zone, there can be attained at most a uranium density of 3.9 grams U/cm.sup.3. U.sub.3 O.sub.8 customarily used as fuel leads to a maximum of 1.9 grams U/cm.sup.3. A production of dispersion nuclear fuel elements with lower enriched uranium for research reactors of average and higher capacity, therefore, is not possible with the known manufacturing techniques since higher volume portions of uranium compounds are necessary in the aluminum matrix.
A further increase of the uranium density in the fuel zone can only be attained by way of a higher volume loading with the uranium compound. In corresponding experiments, it has been shown that from the viewpoint of formability of the picture in the rolling process and maintaining suitable matrix properties in the finished plates, it is possible to increase to about 45 vol. % of uranium compound in the fuel zone.
However, fuel elements produced from such pictures according to the standard technique, consisting of insertion of the pressed, fuel-aluminum-molded body into an aluminum frame, covering with sheets, welding laterally to a roll packet, and rolling cannot be sufficient in an important point in regard to strength quality requirements placed on the reactor operation.
In a plate-shaped fuel element, the geometry of the uranium containing zone is exactly prescribed. Fuel particles outside this rectangular zone, thus in the surrounding aluminum or aluminum alloy zone, are detected in radioscopy and lead to rejection of the plate in question.
While the maintenance of the fuel zone geometry with fuel elements having a volume loading of a maximum of 26 vol. % uranium compound causes no problem, pictures with higher volume loadings lead to fuel elements which show occlusions of small fuel particles outside the specified fuel zone. The cause of this is the relatively brittle material properties of the highly laden picture which leads to abrading or chipping off of small fuel containing particles, which fall between the frame surrounding the picture and the cover sheet and as a consequence are rolled in there. These particles are customarily designated as "white points" because they appear as such in the radioscopy picture of the X-ray examination.
There is described in British Pat. No. 1,071,363 (the entire disclosure of which is hereby incorporated by reference) a process for positioning the picture in the roll packet in which the picture is mounted in the frame with addition sheet insertions and before the rolling is completely enclosed by welding these sheets with the frame cutout. However, this technique is not suitable for the production of fuel elements with higher volume loading of nuclear fuel since it is very expensive and cumbersome. Besides, the described welding of the picture leads to high production waste due to occluded air which cannot escape in the rolls and leads to bonding failures, such as about bubbles in the finished fuel element. Besides, the occurrence of "white points" also cannot be reliably prevented in the fuel-free zone with this technique since these already have been caused in the assembling of picture and frame. The occurrence of uranium at least in the welding seam of the described welded roll packet likewise is not preventable since in rolling these plates the welding seam is also stretched and the "white points" therewith get into the zone specified as uranium free. Such plates, therefore, are unsuitable for insertion in the reactor.
Therefore, it was the problem of the present invention to develop a process for the production of plate-shaped fuel elements for research and material testing reactors with loads of more than 26 vol. % of uranium compounds by mixing the powdery uranium compounds with aluminum powder, pressing to a platelet-shaped molded body, enclosing the molded body in an aluminum jacket, and inserting it into an aluminum plate having a cutout corresponding to the molded body, covering the molded body and the aluminum plate on both sides with two aluminum sheets and rolling this packet.